The invention relates generally to ultrasonic transducers, and, more particularly, to a system for variable multi-dimensional apodization control in an ultrasonic transducer.
Ultrasonic transducers have been available for quite some time and are useful for interrogating solids, liquids and gasses. One particular use for ultrasonic transducers has been in the area of medical imaging. Ultrasonic transducers can be formed of piezoelectric elements or can be fabricated on a semiconductor substrate, in which case the transducer is referred to as a micromachined ultrasonic transducer (MUT). Piezoelectric transducer elements typically are made of material such as lead zirconate titanate (abbreviated as PZT), with a plurality of elements arranged to form a transducer assembly. MUTs are fabricated using various semiconductor substrate materials resulting in a capacitive non-linear ultrasonic transducer that comprises, in essence, a flexible membrane supported around its edges over a semiconductor substrate. By applying contact material to the membrane (or a portion of the membrane) and to the semiconductor substrate, and then by applying appropriate voltage signals to the contacts, the MUT may be energized such that an appropriate ultrasonic wave is produced. Similarly, with the application of a bias voltage, the membrane of the MUT may be used to generate receive ultrasonic signals by capturing reflected ultrasonic energy and transforming that energy into movement of the membrane, which then generates a receive signal. Whether constructed using piezoelectric elements or MUT elements, the transducer assembly is then further assembled into a housing, possibly including control electronics in the form of electronic circuit boards, the combination of which forms an ultrasonic probe. This ultrasonic probe, which may include acoustic matching layers between the surface of the piezoelectric transducer element or elements and the probe body, may then be used to send and receive ultrasonic signals through body tissue.
Regardless of whether the transducer is constructed using piezoelectric elements or MUT elements, in operation it is possible to shape the transmit and receive signals based upon the type of imaging being performed. This is possible because in modern transducers each element in the transducer array is typically connected to the control electronics. In some imaging applications, it is desirable to operate only a portion of the total number of elements in the array at any time. This is referred to as controlling the aperture of the transducer array. The aperture of the transducer array refers to the configuration of the transducer elements that are active at any moment. The electronic control of each element in the transducer allows the transmit and receive signals to be shaped to provide an appropriate signal for the type of imaging being performed. For example, by controlling the transmit energy supplied to some or all of the elements (commonly referred to as xe2x80x9ctransmit beamformingxe2x80x9d) the ultrasonic interrogation pulse sent into the subject can be shaped to provide, for example, high resolution at various depths. Similarly, by electronically altering the receive energy (referred to as xe2x80x9creceive beamformingxe2x80x9d) the received energy can be used to form high quality images at various depths and through various types of tissue.
Various imaging parameters of the ultrasonic transducer can be controlled by varying the transmit energy and operating on the receive energy. For example, by performing transmit and receive beamforming, the elevation and depth of the ultrasonic beam can be varied to provide various lateral and elevation steering angles and various interrogation depths. One manner of controlling the transducer elements is known as xe2x80x9capodization.xe2x80x9d Apodization of an ultrasonic transducer aperture is a gradual reduction of the transmit amplitude and/or receive gain from the center of the aperture to the edges of the aperture with a resultant decrease in beam side lobe levels. In a transmit beam, there is a main energy beam in the direction of interrogation and sidelobe energy located at predictable angles from the main beam direction. These side lobes cause smearing of objects in the image, increase clutter, and reduce contrast. Therefore, it is generally desirable to maximize the transmit energy in the desired direction and reduce the sidelobe energy to levels at which the sidelobe energy will not interfere with the main energy beam. Apodization trades sensitivity and beam width for beam sidelobe levels.
Current ultrasonic transducers have been limited in the amount of apodization control available. Typically, current systems allow apodization control only on one dimension of the transducer. Apodization control in the other dimension (assuming a two-dimensional transducer) is either not performed or is a non-varying function of the first dimension of the transducer. Other systems approximate two-dimensional apodization control by using what is referred to as a xe2x80x9csparse arrayxe2x80x9d in which less than all of the elements in the array are connected to the transmit and receive electronics. Apodization in a sparse array is achieved by decreasing the density of the active transducer elements from the center of the array toward the edges of the array. Unfortunately, the sparse array is constrained so that many elements on the transducer array are unavailable for forming an apodization pattern because they are not connected to the transmitters and receivers. Furthermore, since many of the elements in a sparse array are not connected, the maximum sensitivity of a sparse array will be less than that of a fully sampled array.
In transducer arrangements having fixed or limited apodization control, the tradeoffs between sensitivity, beam width, and beam sidelobe levels cannot be optimized for particular imaging applications. Furthermore, a fixed apodization is optimal only for a particular aperture size of a given transducer. If a different aperture is used, the apodization pattern will be the wrong size. Fixed apodization also fails to allow different apodization profiles to be used for transmit and receive apertures. Fixed elevation apodization restricts the overall aperture apodization to functions that can be separated (i.e. factored) into a product of two functions, one being a function of only the elevation dimension and the other being a function of only the lateral dimension. This is known mathematically as a separable function of the two dimensions of the aperture. Separable apodization functions tend to have beam patterns that concentrate the side lobe energy along the two dimensions by which the function can be separated. It would be advantageous if the side lobe energy could be redistributed in a circularly symmetric manner about the main beam. This would lower the overall side lobe level and even out the influence of the side lobe energy with respect to all areas adjacent to the main beam. Creating a circularly symmetric beam pattern requires a circularly symmetric aperture apodization, which except for a few special cases is not possible using separable functions. Therefore, it would be desirable to have an ultrasonic transducer array in which the apodization function may be a non-separable function of the two dimensions.
When sparse arrays are operated to provide a fixed apodization of the aperture based only on the density of the active elements, they share most of the same drawbacks as transducers having fixed elevation apodization, thus extending the drawbacks to both dimensions of the transducer. Additionally, the amplitude control in a sparse array tends to be crude, relying only on the density of active elements. The transmit and receive amplitudes of the active elements in a sparse array can be controlled, but only those elements actually connected to the transmit/receive electronics can be used, thus constraining the precision with which the apodization pattern can be specified. Furthermore, due to undersampling of the aperture, while sparse arrays tend to improve the side lobe performance of the array at close-in steering angles, the side lobe performance degrades significantly at larger steering angles.
Therefore, it would be desirable to have an ultrasonic transducer array in which variable multi-dimensional apodization control is possible.
Variable multi-dimensional apodization control for an ultrasonic transducer array allows all dimensions of an ultrasonic transducer array to have variable apodization control. The variable multi-dimensional apodization control is applicable to both piezoelectric based transducers and to MUT based transducers and allows control of the apodization profile of an ultrasonic transducer array having elements arranged in more than one dimension.
Other systems, methods, features, and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.